Fixed cutter rotary drill bits for subterranean earth boring have been employed for decades. It is well known that increasing the rotational speed of such drill bit, for a given weight-on-bit (WOB), and subject to the ability of the bit's hydraulic structure to adequately clear formation cuttings from the bit, increases the rate of penetration of the drill string. However, increased rate of penetration of the drill string is limited by the degree to which rubbing contact occurs between a face surface, particularly, the face surface of a blade of the drill bit coming in contact with a bottom hole, or drilling portion of a subterranean formation (i.e., substantially the horizontal facing surface of the bottom hole portion) while drilling.
Another recognized concern is that damage to cutting elements, commonly polycrystalline diamond compacts (PDC), may occur at higher rates of penetration, particularly at higher rotational speeds, and is at least in part attributable to a phenomenon known as “whirl” or “bit whirl.” Radially directed centrifugal imbalance forces exist to some extent in every rotating drill bit and drill string. Such forces are in part attributable to mass imbalance within the drill bit and in part to dynamic forces generated by contact of the drill bit with the formation. In the latter instance, aggressive cutter placement and orientation creates a high tangential cutting force relative to the normal force applied to the bit and aggravates the imbalance. In any event, these imbalance forces tend to cause the drill bit to rotate or roll about the bore hole in a direction counter to the normal direction of rotation imparted to the bit during drilling. This counter-rotation is termed “whirl,” and is a self-propagating phenomenon, as the side forces on the bit cause its center of rotation to shift to one side, after which there is an immediate tendency to shift again. Since cutting elements are designed to cut and to resist impact received in the normal direction of bit rotation (clockwise, looking down a drill string), contact of the cutting elements with the bore hole wall in a counter-clockwise direction due to whirl can place stresses on the cutting elements beyond their designed limits.
One solution to the problems caused by bit whirl has been to focus or direct the imbalance forces as a resultant side force vector to a particular side of the bit via changes in cutting element placement and orientation and bit mass location, and to cause the bit to ride on a low-friction bearing zone or pad on the gage of that side of the bit, thus substantially reducing the drill bit/bore hole wall tangential forces which induce whirl. This solution is disclosed in U.S. Pat. Nos. 4,982,802; 4,932,484; 5,010,789 and 5,042,596, all assigned on their faces to Amoco Corporation of Chicago, Ill.
The above-referenced U.S. Patents conventionally require that the low friction bearing zone or pad on the gage and adjacent bit profile or flank be devoid of cutting elements and, indeed, many alternative bearing zone configurations are disclosed, including wear coatings, diamond stud inserts, diamond pads, rollers, caged ball bearings, etc. It has also been suggested by others that the bearing zone on the bit gage may include cutting elements of different sizes, configurations, depths of cut and/or rake angles than the cutting elements located in the cutting zone of the bit, which extends over the bit face from the cutting elements thereof outwardly to the gage, except in the flank area of the face adjacent the bearing zone. However, as represented in the prior art, such bearing zone cutting elements undesirably generate lesser cutting forces than the cutting elements in the cutting zone of the bit so that the bearing zone will have a relatively lower coefficient of friction. See U.S. Pat. No. 4,982,802, Col. 5, lines 29-36; U.S. Pat. No. 5,042,596, Col. 4, lines 18-25. Furthermore, while the prior art provides for focusing or directing the imbalance forces as a resultant side force vector toward a particular side of the bit, it does so by compromising cutting aggressiveness of the bit, particularly affecting the placement and aggressiveness of cutting elements. Moreover, while the above-referenced patents reduce tangential forces, which are generally noted to induce whirl, they do not protect the cutting elements from damage as a result of the impact loads caused by vibrational instabilities commensurate with bit whirl, particularly when drilling in harder subterranean formations.
In order to mitigate the damage upon the cutting elements caused by side impact forces, conventional wisdom has been to direct the imbalance force, i.e., the resultant side force vector, of the bit toward the center of the bit blade and trailing bearing surface of a bit blade or toward the gage region of a particular bit blade, which undesirably limits cutter placement and configuration and other features of the design of the bit. Damage to the cutting elements may also be mitigated by increasing the circumferential width of the bearing surface, which undesirably reduces the hydraulic cross-section available for the junks slot, thus reducing hydraulic flow of drilling fluid and potentially decreasing the volume of cuttings which may be carried therethrough by the drilling fluid. In order to improve the stability of the bit while militating against damage, the bearing surface has been extended across the width of one or more channels between blades. Such bits are known as so called “steering wheel” drill bits and generally include fins or cylindrical portions that extend the bearing surface circumferentially about the gage region of the drill bit as shown and described in U.S. Pat. Nos. 5,671,818, 5,904,213 and 5,967,246. While these so called “steering wheel” drill bits may increase stability by militating against vibrational instabilities and enhance the ability of such bits to hold bore hole gage diameter, such drill bits undesirably increase the outer perimeter surface of the bit bearing on the bore hole side wall, making directional drilling more difficult. Furthermore, the configuration of such so called “steering wheel” drill bits also undesirably reduces the available hydraulic cross-section of the junk slots and may restrict removal of formation cuttings from the drill bit face by substantially circumscribing the flow channels provided by the junk slots. In addition, the configuration of the steering wheel drill bits impedes tripping the drill bit in and out of the bore hole, and may cause swabbing (removal of formation material from the bore hole side wall) during tripping.
Another solution to mitigate the damage upon the cutting elements caused by side impact forces is provided in U.S. patent application Ser. No. 11/865,296, titled “Drill Bits and Tools For Subterranean Drilling,” filed Oct. 1, 2007, and U.S. patent application Ser. No. 11/865,258, titled “Drill Bits and Tools For Subterranean Drilling,” filed Oct. 1, 2007, which are owned by the assignee of the present invention, and which disclosures are incorporated herein in their entirety by reference.
While the above mentioned solutions have reduced, in some aspects, instability of the bit due to bit whirl in order to increase rotational speed and, resultantly, rate of penetration, the face surface (particularly the face surfaces of the blades) of the bit limits rate of penetration due to rubbing contact with a subterranean formation. The face surface of each blade has a continuous contoured radially and laterally extending profile, or engagement surface, that is substantially attributable to cutter profile design and structural support of the cutting elements. In other instances, the face surface of each blade has a continuous contoured radially and laterally extending profile, or engagement surface that is extended rotationally to accommodate the greater structural extent required by the bearing surface of the gage pads required for increased stability.
Accordingly, it is desirable to provide improvements for a drill bit to increase rate of penetration undiminished by the extent of rubbing contact between the drill bit and a subterranean formation. Moreover, it is desirable to provide improvements for a drill bit to maintain or enhance stability by reducing lateral motion affected by bit whirl while providing increased rate of penetration undiminished by the extent of rubbing contact between the drill bit and a subterranean formation.